I know that the structures created by barrier have a toughness of 10 but how much armor is given to someone taking cover behind it?

Well, there are two types of armor from cover.

If it is an Area Effect attack, then the barrier would provide a bonus equal to the cover modifier provided.

If it is an attack shooting through an obstacle (per pg. 75 of SWD), then (barring trappings) the Armor bonus would be the same as the barrier's Toughness._________________Clint Black
Savage Worlds Core Rules Brand Manager